what exactly is the Nafka mina between R yonoson & r. yoshiya
Whenever Tanaim reach the same Halachic conclusion but Darshen different Pesukim to do so, this will usually cause an argument between them down the line, as they each now have different Pesukim that are open for Derasha and different Pesukim that are closed for Derasha. The Gemara will often pursue this train of thought, as it does in our Gemara, by saying what does A do with the other Tana's Pasuk? What does the other Tana do with B's Pasuk? Tosfos (24b, DH "v'Rebbi Yonasan") indeed asks that the Gemara seemingly does not have an answer to how Rebbi Yonasan and Rebbi Yoshiya know the Halachos derived from Pesukim which the other already used for Arusah and Shomeres Yavam, which might indicate that they argue about those Derashos. Although some commentaries answer Tosfos' question (see Maharsha), it is probable that they argue regarding some Derashah down the line.
However, even if they would not, it is still important for Rebbi Yonasan and Rebbi Yoshiyah to tell us the true meaning of the Torah, whether each Pasuk is talking about either Arusah or Shomeres Yavam.
All the best,